1. Technical Field
This application relates to a level switch which detects that the surface of a material contained within a tank has reached the level of a sensor belonging to the level switch.
2. Description of the Prior Art
Level switches of this kind are applicable e. g. in silos, storage tanks or reservoirs, which may be open or closed, exposed to atmospheric pressure, to a variable or a steady pressure. Materials contained therein may be quite diverse: liquid, granular or powdery materials of different density, viscosity and adherence, pure or mixed with rigid particles of different size, with air bubbles, with a surface foam, homogeneous or inhomogeneous, with steady or variable electric properties. The application range of a single known level switch is rather restricted and usually the level switch must be adjusted to the material contained in a tank.
The level detection is based on different principles. There are known mechanical, electromechanical and hydrostatic level switches, ultrasonic level switches, capacitive and conductive level switches, microwave level switches, optical and radiometric level switches.
Mechanical and electromechanical level switches are suitable for liquids only. Owing to a float and transmission rods, they are sensitive to deposits, to rigid particles contained in a liquid, to turbulences and to foam on the liquid surface.
Capacitive level switches are universal to a greater degree. The capacity of the measuring capacitor within a sensor varies according to the surface level displacement of the material contained in a tank since the permittivity value of the material differs from 1.
There has been known a capacitive sensor with a circuitry (FTC 968, described in Handbuch fur Ingenieure; Sensoren, Me.beta.aufnehmer, Neue Verfahren und Produkte fur die Praxis, 2nd edition, pages 521 to 529, Expert Verlag, Ehningen bei Boblingen (1988)) to sense the filling level of a tank containing fine-granular bulk material (grain size up to 10 mm) with the permittivity value exceeding 1.6. The sensor is designed so that the interfering influence due to a sticking material deposit is eliminated automatically. The sensor is made of a hollow plastic cylinder closed at one end, within which a measuring electrode is mounted on the cylinder base and also partially on the jacket. In addition, on the inner wall of the sensor cylinder jacket a screening electrode and a grounded terminal ring electrode are mounted. On the screening electrode a circuitry multivibrator at a constant frequency of 0.5 MHz builds up a potential, which varies with a phase delay with respect to the measuring electrode potential. Thereby it is achieved that electric field lines built up between the measuring electrode and the terminal ring electrode are pushed out by the screening electrode. Therefore the electric field lines project from the material sticking on the sensor. The described level switch is suitable for a dry bulk material, however, it is not suitable for an electrically conductive moist bulk material.
Further, a capacitive level switch (model 23 and 25 of company VEGA, Germany) has been known which is also suitable for electrically conductive and heavily sticking materials. A completely insulated rod sensor comprises a guard screen electrode which compensating the effect of a layer of a sticking material. The guard screen electrode shunts the undesired electric current flowing through this layer from the sensor tip to the fixing part.
All known capacitive level switches must be adjusted to the material contained in a tank, and their read-outs are not appropriate any more as soon as the material parameters have been changed. Likewise, there has been known no level switch, by which a change from an electrically conductive material to an electrically non-conductive one, from a sticking material to a non-sticking one and so forth could be accomplished without a former adjustment. Thus by known level switches the problem of detecting a drop of a material having the electrical conductivity below 0.1 mS/cm and not sticking to the level switch sensor can still be solved, whereas this is for a sticking material not feasible by any level switch, neither by a capacitive nor by a conductive one. In such extreme case an ultrasonic or radiometric level meter are applied, which, however, is an expensive and demanding solution and not always feasible as regards design or otherwise.
Known conductive level switches are suitable for materials with the electrical conductivity above 3 .mu.S/cm and must be adjusted to respective material contained in a tank. There is applied a sensor with mutually distant electrodes surrounded by the material contained in the tank, one of the electrodes being connected to an oscillator of frequency in the range between 3 kHz and 4 kHz. The oscillator feeds a bridge circuit, in whose one arm the level switch sensor and in the other arm a potentiometer to adjust to the material contained in the tank are connected. The bridge circuit signal is conducted to a comparator and thereafter, across a filter suppressing interfering signals, to an output amplifier. The electrical resistance of the material between the sensor electrodes is representable as a voltage drop on a constant resistance comprised in the circuit, which voltage drop is conducted further to a comparator. In the comparator the switching operating point must be set.
There is known a level switch (VEGATOR 261 A of VEGA, Germany) in which uniquely among level switches of this kind no adjustment is needed when the material contained in a tank is changed. The sensor is provided with three ring electrodes separated from each other and having different surfaces, which electrodes are connected to an oscillator of a constant frequency of 4 kHz. A balance of resistances of the material between electrodes renders it possible that the level switch can operate in materials with a low electrical conductivity between 1 .mu.S/cm and 15 .mu.S/cm. However, this level switch is not suitable for materials, like distilled water, with very low electrical conductivity.